What do I say in a first message?
Ask about one specific thing on their profile. That is the whole technique.
"Hey" is not a message. It is a request for the other person to do the work of starting, and on a busy profile it arrives thirty times a week. It gets ignored for the same reason a blank envelope gets ignored.
The reliable opener
Pick one specific thing from their profile and ask about it. Not the photos — the words. A prompt answer, a line in their bio, the city they mentioned. "You said you moved to Lyon last year. Was that for work or to get away?" takes ten seconds to write and is impossible to ignore, because it proves you read.
Specificity beats cleverness. A funny line that could have been sent to anyone reads as a copy-paste, and it usually is one. A boring question about something only they wrote does not.
Ask something they can answer badly
Good openers have low stakes. "What is the best thing you cooked this month?" can be answered with one word or three paragraphs, and neither feels wrong. "What are you looking for?" is a job interview, on the first line, before anyone has decided they like you.
What not to lead with
Do not open with anything about their body, their surgeries, their hormones, or how they looked before. This is not politeness — it is the single most common reason people stop replying. If you would not ask it of a cis stranger in a queue, it is not a first message.
Do not open with a compliment about how well they pass. It reads as a grade, and it tells them you were assessing. See passing for why that lands the way it does.
Do not open with your own history unprompted. "I started T two years ago" is fine on your profile and strange as an opening line: it asks them to respond in kind before they know you.
The advantage you have here
On a T4T app you can skip the part where you explain yourself. Nobody needs your transition summarised, nobody is deciding whether you count. That frees the first message to be about the ordinary thing it should be about: whether the two of you are interesting to each other.
Use it. The most common mistake trans people make on trans-only apps is opening with the explanation they have had to give everywhere else.
Length, and when to stop
Two or three sentences. Long enough to say something, short enough that a reply costs nothing.
If there is no answer after one message, leave it. A second message asking why they did not reply converts a silence into a bad experience, and it is the fastest way to get reported. Silence is an answer, and it is allowed to be one.
If you are the one being messaged
You owe no reply. Not to a good message, not to a long one, not to someone who was polite. Unmatching without explanation is a normal use of the app, not a rudeness.